Serine proteases are enzymes which, amongst other things, play an important role in the blood coagulation cascade. Members of this group of proteases are for example thrombin, trypsin, factors VIIa, IXa, Xa, XIaa, XIIa, and protein C.
Thrombin is the serine protease which regulates the last step in the coagulation cascade. The prime finction of thrombin is the cleavage of fibrinogen to generate fibrin monomers, which form an insoluble gel by cross-linking. In addition, thrombin regulates its own production by activating factors V and VIII earlier in the cascade. It also has important actions at cellular level, where it acts on specific receptors to cause platelet aggregation, endothelial cell activation and fibroblast proliferation. Thus thrombin has a central regulatory role in haemostasis and thrombus formation Since inhibitors of thrombin may have a wide range of therapeutical applications, extensive research has been performed in this are& In the development of synthetic inhibitors of serine proteases, and more specifically of thrombin, the interest in small synthetic peptides that are recognized by proteolytic enzymes in a manner similar to that of natural substrates, has increased. As a result, new peptide-like inhibitors have been prepared, such as the transition state inhibitors of thrombin. The search for more effective and more selective thrombin inhibitors continues unabated in order to obtain thrombin inhibitors which can be administered in lower dosages and which have fewer and less severe side effects. Furthermore, special attention is paid to oral bioavailabilirty. Potent intravenous thrombin inhibitors are clinically effective in acute care settings requiring the treatment of thrombin-related diseases. However, particularly the prevention of thrombin-related diseases such as myocardial infarct, thrombosis and stroke require long-term therapy, preferably by orally dosing an anticoagulant.
Most of the peptide-like thrombin inhibitors disclosed in prior publications contain side chains of arginine. A problem of the those arginine containing thrombin inhibitors is that they have low oral bioavailabldity. A number of thrombin inhibitors contain lysine side chains instead of arginine have been reported in literature. The lower basicity of lysine, relative to argiine, was expected to result in increased oral bioavailability. Examples of lysine containing thrombin inhibitors are the inhibitor N--Me--D--Cha--Pro--Lys--COOH and derivatives thereof, disclosed by Jones et al., J. Enzyme Inhibition, 9 (1995), 43-60, and the inhibitors N--Me--D--Phe--Pro--Lys--X, X being a carboxamide or carboxylic acid, disclosed by Lewis et al., Thrombosis and Haemostasis, 74(4) (1995), 1107-12. However, the expected improvement was not observed, the oral bioavailabilities of several lysyl thrombin inhibitors being similar to that reported for inhibitors having an arginine side-chain (Lewis et al., Thrombosis and Haemostasis, 74(4) (1995), 1107-12). Other thrombin inhibitors are disclosed in WO 94/25051 wherein the lysine or arginine side chain is replaced by aminocyclohexyl moieties which may be regarded as lysine isosters.